Fluid catalytic cracking or FCC is effected by contacting the hydrocarbons in a conversion zone with a catalyst which is made up of a fine particulate material. Opposite to the hydrocracking, the catalytic cracking is effected in the complete absence of added hydrogen or hydrogen consumption. Generally, the most common feeds submitted to the FCC process are those refinery streams originating from side cuts of vacuum towers, called heavy vacuum gasoil, or heavier than those, originating from the bottom of atmospheric towers and called atmospheric residue, or still, mixtures of these streams. These streams, of density typically in the range of 8 to 28.degree. API, should be submitted to a chemical process such as a catalytic cracking, to have their composition deeply altered, so as to be converted into lighter hydrocarbon streams, of higher economic value.
During the cracking reaction, substantial amounts of coke, by-product of the reaction, are deposited on the catalyst. Coke is a high-molecular weight stuff, made up of hydrocarbons which typically contain of from 4 to 8 weight % of hydrogen in their composition. The coke-recovered catalyst, usually called spent catalyst by the experts, is continually removed from the conversion zone and replaced by catalyst essentially free of coke from the regeneration zone. The burning of the coke deposited on the surface and in the pores of the catalyst is effected in the regeneration zone, in a regeneration vessel kept at high temperature. Eliminating coke by combustion allows the recovery of the catalyst activity and releases heat in an amount which is sufficient to attend the thermal needs of the catalytic cracking reactions. The fluidization of the catalyst by gaseous streams makes possible the transport of catalyst between the conversion zone and the regeneration zone and vice-versa. The catalyst, besides its essencial function which is to promote the catalysis of the chemical reactions, is also the means for transporting heat from the regenerator to the conversion zone.
The state-of-the-art is abundant in descriptions of processes for cracking hydrocarbons in a fluidized stream of catalyst, with catalyst transport between the conversion zone and the regeneration zone, and burning of coke in the regenerator. In spite of the long existence of FCC processes, techniques are continually sought which would still improve the process, in order to increase the production of higher priced products such as gasoline and LPG. Generally, it can be stated that the main goal of the FCC processes is the maximization of the production of these more valuable products.
Basically, the maximization of more valuable products is obtained by two methods. One method comprises the increase in the so-called conversion, which corresponds to reduction in the production of heavy products such as the clarified oil and the light cycle oil. Another method comprises reducing the yield in coke and fuel gas, that is, the selectivity of the process to these products is reduced. The consequence of the lower production of coke and fuel gas is the increased production of gasoline and LPG, this meaning increased selectivity of the process to these valuable products. Further benefits are the use of smaller air blower and wet gas compressor which are large dimension, high-consuming energy machinery and which generally determine the limits of the capacity of the FCC units.
It is well-known that an important feature of the FCC process is the initial contact of the catalyst with the feed, this having a paramount influence on the conversion and selectivity to noble products. In the FCC process, the pre-heated hydrocarbon feed is injected near the bottom of a conversion zone or riser, which is an extended, vertical pipe. Generally the height of this pipe is of from 20 to 40 meters while its diameter is of from 0.5 to 1.5 meters. In the riser the feed contacts the flow of regenerated catalyst from which it takes heat in an amount which is sufficient to atomize the feed and provide for the thermal duty of the endothermic reactions which predominate in the process.
After the riser, where the chemical reactions occur, the spent catalyst, having coke deposited on its surface and pores, is separated from the reaction products and sent to the regenerator for burning coke in order to have restored its activity and generate heat which, when transferred from the catalyst to the riser, will be used in the process.
The conditions found at the point where the feed is introduced into the riser determine the products formed in the reaction. At this point there is the initial mixture of the regenerated catalyst and the feed, with the heating up to boiling point and the vaporization of most of these constituents of the feed. The overall residence time of the hydrocarbons in the riser is of just 2 seconds. In order that the catalytic cracking reactions proceed, it is necessary that the vaporization of the feed in the mixing region with the catalyst occur in a few miliseconds, so that the molecules of the vaporized hydrocarbons can contact the catalyst particles. The size of the catalyst particles is around 60 micra and the hydrocarbon molecules permeate through the pores of the particles so as to be affected by the acid sites of the catalyst which ultimately cause the catalytic cracking. In case the quick vaporization is not attained, the liquid fractions of the feed are thermally cracked.
It is well-known that the thermal cracking leads to by-products such as coke and fuel gas, chiefly when residual feeds are cracked. Coke, besides its low intrinsic value, plugs the catalyst pores. Thus, the thermal cracking at the bottom of the riser undesirably competes with the catalytic cracking, which is the actual goal of the process.
On the other hand, the quick vaporization of the feed will be more easily obtained if the feed is suitably atomized, so as to form a fine spray on the catalyst phase. Various models of injectors designed to inject feed into the riser have been developed in order to obtain this spray. There is evidence that the higher the temperature of the feed in the atomizer, the larger will be the surface area of the droplets of the spray and thus the larger will be the contact area between the feed and the catalyst, with significant influence on the ease of vaporization. It can be demonstrated that, for the residual feeds used in the FCC process and for the range of temperatures which is practiced, the increase in contact area with the use of higher feed temperatures can attain 30%.
In order to maximize conversion of the feed it is usual to seek for maximum removal of coke from the catalyst in the regenerator. The combustion of coke may be obtained under partial or total combustion.
Under partial combustion, the gases which are produced by the combustion of coke are made up chiefly of CO.sub.2, CO and H.sub.2 O and the coke content in the regenerated catalyst is of the order of 0.1 to 0.2 wt %.
Under total combustion, carried out in the presence of a larger excess of oxygen, practically all the CO produced in the reaction is converted into CO.sub.2. The oxidation reaction of CO into CO.sub.2 is strongly exothermic, so that total combustion occurs with large heat release which results in very high regeneration temperatures. However, total combustion yields a catalyst which contains less than 0.07% and preferably less than 0.05 wt % of coke, which renders it more advantageous as compared to the partial combustion, besides the fact that a high cost boiler for the combustion of CO can be dispensed with.
The increase of coke in the spent catalyst results in an increase of coke under combustion in the regenerator by mass unit of circulated catalyst. Heat is removed from the regenerator in conventional FCC units in the combustion gas and chiefly in the stream of regenerated hot catalyst. An increase in the coke content on the spent catalyst increases the temperature of the regenerated catalyst as well as the difference in temperature between the regenerator and the reaction vessel (reactor). A reduction in the flow rate of regenerated catalyst to the reactor, called catalyst circulation, is therefore necessary in order to provide for the thermal duty of the reactor and keep as such the reaction temperature. However the lower catalyst circulation rate required by the larger difference in temperature between the regenerator and the reactor causes a lowering of the catalyst/oil ratio and reduction of the conversion.
Thus, the catalyst circulation from the regenerator to the reactor is a function of the riser thermal duty and of the temperature which is established in the regenerator, this being a function of the coke production. In view of the fact that the coke which is generated in the riser is affected by the catalyst circulation itself, it can be concluded that the process of catalytic cracking works under a heat balance regime, which, based on the reasons set forth hereinbefore, renders undesirable the operation under a very high regeneration temperature.
There are further limitations to the temperatures which can be tolerated by the FCC catalyst without negatively affecting its activity. Generally, with the modern FCC catalysts, the regenerator temperatures and thus the temperatures of the regenerated catalysts are kept below 760.degree. C., preferably below 732.degree. C., since the loss in activity would be severe beyond this figure. A desirable operation range is between 685.degree. C. and 710.degree. C. The lower limit is controlled mainly by the need of securing suitable coke combustion. For units processing atmopheric residues, the regenerator, were it not for a heat-removing system, would operate at temperatures in the reange of 870-980.degree. C. for most cases.
Therefore, the cooling of the regenerator aims at bringing its temperature to acceptable values from the point of view of the catalyst as well as from the equipments involved and as regards of the establishment of a catalyst circulation of a commercially acceptable range.
This approach is used in FCC units which crack heavy feedstocks such as atmospheric residues or their mixtures with the heavy vacuum gasoil. The cooling of the regenerator is imperative when the available feeds are residual feeds with high coke output and when the regeneration is effected under total combustion.
Total combustion is being increasingly practiced in the field, since among other advantages it leads to a rather low coke content on the catalyst, lower than 0.05 wt %, which improves the conversion.
It should be understood that the processing of increasingly heavy feedstocks, the tendency of such feeds to increase the coke production as well as the operation under total combustion require that catalyst coolers be installed in order to keep the temperature of the regenerator under acceptable limits. Normally the catalyst coolers remove heat from a catalyst stream from the regenerator so that the catalyst stream which returns to the regenerator is substantially cooled.
The cooling of the catalyst has been the object of numerous patents. There are coolers which are internal to the regenerator, their operation being effected through coils, in the interior of which a cooling fluid circulates, see for example U.S. Pat. No. 2,819,951. There are also descriptions of catalyst coolers which are external to the regenerator. U.S. Pat. No. 2,970,117, for example, teaches that the return flow rate of the cooled catalyst to the regenerator can be controlled by means of the regenerator temperature.
A further possibility for removing heat from the regenerator consists in cooling the catalyst which is sent to the riser. This renders the catalyst cooler in the portion of riser which precedes the introduction of feed, with the consequence that the catalyst circulation is increased and the thermal charge of the regenerator is more thoroughly removed, so that the regenerator is cooled.
However, in case there is the need of an additional increase in catalyst circulation so as to achieve higher conversion into noble products, the system will suffer from an excessively low regeneration temperature which renders this operation unsatisfactory. The increase in circulation is desirable in the cracking of residual feeds, since these are feeds of difficult crackability.
In order to prevent that the regenerator temperature be reduced to unacceptable values caused by the increase in catalyst circulation (caused by the heat balance effect as discussed hereinbefore) the temperature of the feed to the riser can be compatibly raised. Under this condition, the thermal duty of the riser is kept similar to the previous condition except that the catalyst is colder and the feed is hotter. This condition does not provide the optimum catalyst circulation, however the difference in temperature between the streams of catalyst and feed is substantially reduced.
The reduction in this driving force of temperature, associated to the increased ease of atomization of the heavy constituents of the feed, made possible by the higher initial temperature and better atomization of the feed, diminishes the occurrence of thermal cracking reactions which yield coke and fuel gas, in the region of mixing between feed and catalyst. These conditions favor the catalytic route which is the basic goal of the FCC process, while minimizing the thermal cracking.
Catalyst can be cooled by using water, however this technique has drawbacks such as the overload of equipments such as the riser, cyclones, the fractionating tower top condensers and the acid waters system; the increased deposition of ammonium salts in the fractionating tower, increased volume of waste waters and energy loss for vaporizing the water which is later recondensed without heat recovery.
In order to eliminate the drawbacks of cooling the riser with the aid of water, a catalyst cooler may be used. The catalyst from the regenerator is cooled in a high-pressure steam generator and from there is directed to the riser. Thus an energetic otimization is created by means of high pressure steam generation. The surplus in steam generation means substantial energy savings, as compared to the injection of water.
U.S. Pat. No. 4,396,531 teaches, in a process for regenerating the FCC catalyst contaminated by coke, an external cooler used to cool the stream of regenerated catalyst to the riser. In the cooler, the hot regenerated catalyst is made to contact under conditions of heat exchange a cooling fluid which is boiler water to yield a relatively cold catalyst, the catalyst being kept in the cooling zone as a dense phase fluidized bed where a fluidizing gas is circulated. It is alleged that the flow rate of the catalyst stream to the cooling zone is adjusted so as to render possible the optimization of the combination of variables which comprises the amount of heat to be removed; the goal of passivation of contaminating metals such as nickel and vanadium; the content of non-condensible gases, which are entrained with the catalyst to the riser. It is stated that the reaction temperature is controlled by means of the flow of relatively cold regenerated catalyst to the reaction zone.
It should be emphasized that by cooling the catalyst directed to the riser, the objective in U.S. Pat. No. 4,396,531 is the cooling of the regenerator by the increase in the catalyst circulation. The main goal of this patent is not the reduction of the thermal cracking with the aid of the cooling of the catalyst which is being directed to the riser and the corresponding heating of the feed, in spite of the fact that is in part achieved. Apparently the teachings of this patent are a counterpart to the numerous patents directed to coolers which return the catalyst to the regenerator, alleging modifications in the thermal properties of the fluids involved in the heat exchange.
When aiming at complying with the specific goal of cooling the regenerator to the required temperature for the adequate operation of the regenerator, the teachings of U.S. Pat. No. 4,396,531 do not lead to the adequate and independent control of the regenerator temperature and of the catalyst circulation, required by the heat balance of the FCC unit, as discussed hereinbefore. U.S. Pat. No. 4,396,531 does not consider the advantage of the adequate interference in the heat balance of the FCC unit. A neat evidence of this is that the control of the reaction temperature is effected by means of the variation of the catalyst flow rate from the cooler to the riser by actuation of valve (21) placed in the corresponding standpipe (5). In U.S. Pat. No. 4,396,531 one cannot find a counterpart which could allow the independent control of the catalyst circulation to the riser, and therefore the catalyst/oil ratio.
Therefore, as regards to the heat balance in a FCC unit, there are several simultaneous parameters to be met: to cool the regenerator keeping its temperature at an adequate value, besides maintaining the catalyst circulation and therefore the catalyst/oil ratio at adequate values which implies obtaining the desired reaction temperature. Thus, U.S. Pat. No. 4,396,531 does not contemplate a degree of freedom which would make the catalyst/oil ratio an independent variable. This is because this patent is not concerned by the heat balance aspects of the unit nor by the advantage of having an independent control of the temperature of the catalyst which contacts the feed and of the temperature of the feed itself.
U.S. Pat. No. 4,234,411 teaches, in a FCC process, a method for the control of the flow rate of two or more regenerated catalyst streams towards the riser. According to the suggested method, the feed to be cracked in the riser is made to contact a first portion of regenerated catalyst where the catalyst flow rate is a function of the temperature of the mixture of this catalyst stream and the feed; then this mixture of feed and catalyst is made to contact a second portion of regenerated catalyst whose flow rate is controlled by the final reaction temperature. In this patent, in spite of the regenerated catalyst being introduced in the riser in two points, in both points the catalyst is at the same temperature. It is the flow rate of catalyst which is varied as a function of the reaction temperature. This patent does not consider in any way altering the heat balance of the unit; it does not take advantage of the existence of cooled catalyst in the region of contact with the feed and of the heated feed. Further, by not acknowledging the principle of the heat balance of the unit this patent does not lead to the independent control of the regenerator temperature, of the feed temperature and of the catalyst/oil ratio.
U.S. Pat. No. 4,257,875 is analogous to U.S. Pat. No. 4,234,411 in that it teaches the introduction of regenerated catalyst in more than one point of the riser. In the described process, the first stream of regenerated recycled catalyst is introduced at a flow rate which is sufficient to bring the temperature of the mixture with feed up to the range of 454.degree. C. and preferably beyond 510.degree. C. so as to atomize most of the distillable portion of the feed. This patent presents a table where the temperature of the feed and the catalyst/oil ratio are the same for the state-of-the-art and the patent, indicating that no modification has been introduced into the heat balance of the unit.
U.S. Pat. No. 5,451,313 teaches a FCC process where the severity of the process is reduced and the feed dispersion and the contact with catalyst are improved by circulating spent catalyst together with regenerated catalyst. Spent and regenerated catalyst are combined so as to near or attain the heat balance between the two catalyst streams before the contact of the catalyst mixture with the feed. The temperature resulting from the mixture between the spent and regenerated catalyst is less than the temperature of regenerated catalyst. It is alleged that the reduced temperature of the catalyst particles together with the increased volume of catalyst promotes a more uniform heating of the feed as well as a better dispersion of feed in the catalyst.
However, there are three main drawbacks which severely restrict the use and benefits of U.S. Pat. No. 5,451,313.
As regards the first of these drawbacks, it is found that when spent catalyst is recycled to the riser, the overall volume of catalyst to be contacted with the feed is increased. This causes that the contact of the feed with the particles of regenerated catalyst is reduced, the regenerated catalyst being the effective catalyst which promotes the reactions of the catalytic cracking. On the other hand, the spent catalyst, having coke deposited on its particles, is a low-activity catalyst. This reduces the conversion of the unit. Besides, the spent catalyst is more coke-selective, since the reactions of coke production are knowlingly catalyzed by the presence of coke, thus the production of undesirable coke is increased. Therefore, the use of a portion of spent catalyst, which induces thermal instead of catalytic cracking, reduces the conversion of the process and worsens its selectivity, this lowering the economics of the process. The process taught in U.S. Pat. No. 5,451,313 could be adequate only for the cracking of light or hydrotreated feeds, having extremely low coke production. Thus, this kind of process is not indicated for the cracking of heavy feedstocks, of increasingly use in the FCC process, these feeds being of difficult crackability, high coke production and which result in catalysts which are highly contaminated by the presence of metals.
A second drawback which limits the use of U.S. Pat. No. 5,451,313 relates to the use of the large recycle flow rate of spent catalyst which is required to the mixture of catalyst at the base of the riser. The fact of it being a recycle, leads to over-dimensioning of the riser, the cyclones, the stripper and the standpipes. Those are large-dimension equipments which bring huge additional costs to a FCC unit. Besides, as a consequence of the increase in the stripper size, it is necessary to increase the flow rate of stripping steam so as to obtain an adequate speed in that equipment. Therefore, operation costs are equally increased.
A third and by no means not less important drawback of the technique addressed in U.S. Pat. No. 5,451,313 relates, as discussed for other patents hereinbefore, to the fact that this patent does not mention aspects relating to the heat balance of the unit. Actually, by using a recycle of spent catalyst to the riser, the heat balance is not altered, since the catalyst is recycled to the beginning of the riser at a temperature which is practically the same as the temperature of the mixture of catalyst and feed at the end of the riser. Therefore, in practical terms, the stream of spent catalyst does not contribute to add or withdraw heat from the riser. In view of the fact that this stream does not alter the heat balance, whenever there is a modification in the feed temperaure, there will be as a consequence a modification in the flow rate of regenerated catalyst to the riser and/or in the regenerator temperature. For example, if there is an increase in the feed temperature there will be a lesser circulation of regenerated catalyst to the riser, as a consequence of the lesser thermal duty of the riser. This occurs even if the temperature of the regenerator is kept at a constant value by means of a catalyst cooler. Therefore, U.S. Pat. No. 5,451,313 cannot benefit from the increase in feed temperature without provoking a reduction in the circulation of regenerated catalyst. The counterpart for keeping the circulation of regenerated catalyst would be by intervening in the heat balance by means of a catalyst cooler. This would entail a reduction in the regenerator temperature, which would have a negative effect on the regeneration. Therefore, the teachings of U.S. Pat. No. 5,451,313 do not allow that the feed temperature, the regenerator temperature and the circulation of catalyst be independent parameters.
Therefore, the patent literature does not teach nor suggests the concept of the present invention, that is, the combination of a stream of hot, regenerated catalyst at the temperature of the regenerator, and a stream of cooled regenerated catalyst, such mixture of catalyst streams being made to contact the feed to be cracked, the catalyst cooler being used to cool the regenerator bed as well as the regenerated catalyst designed to crack the hydrocarbon feed in the riser.
The inventive combination of streams of regenerated catalyst at different temperatures, both being controlled, leads to a mixture of regenerated catalyst having a temperature which is arbitrarily set by the operator of the unit. This feature makes it possible the independent control of the circulation of regenerated catalyst, dissociated from the feed temperature, the regenerator temperature and the reaction temperature as discussed in detail hereinbelow. The inovative action on the heat balance of the unit introduces in the technique a revolutionary concept of independence between the main variables which affect the heat balance of the process of fluid catalytic cracking.
Thus, the need which exists in the art, of a FCC process for heavy feedstocks which would operate under a regime of heat balance, at low cost and yielding high amounts of noble products and low amounts of fuel gas and coke, is provided for by the process which is described and claimed in the present application.